1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal mixture and a liquid crystal device using the same.
2. Description of the Related Art
Liquid crystal display devices which have been most widely used in recent years employ a nematic phase of liquid crystal. In twisted nematic (TN) liquid crystal display devices, it is difficult to realize a display with a large capacity, e.g., 2000.times.2000 lines, since contrast decreases with the increase in the number of lines. In order to improve the TN liquid crystal display devices, supertwisted nematic (STN) liquid crystal display devices and double layer supertwisted nematic ( DSTN ) liquid crystal display devices have been developed. However, in these devices, contrast decreases and response time becomes longer with the increase in the number of lines; therefore, display capacity is now limited to about 800.times.1024 lines.
Active matrix type liquid crystal display devices have been developed to realize a display with a large capacity, e.g., 1000.times.1000 lines. These devices also have disadvantages such as many steps for prolonged production, a decrease in yield, and high production cost.
In recent years, in addition to liquid crystal display devices employing a nematic phase, those employing a smectic phase have been extensively studied. In particular, a ferroelectric liquid crystal display device has been considered to be promising (N. A. Clark et al., Appl. Phys. Lett., 36, 899 (1980)). The ferroelectric liquid crystal display device employs a memory property of a chiral smectic C phase, a chiral smectic I phase, and the like. Because of this, the ferroelectric liquid crystal display device enables a display with a large capacity accompanied by the decrease in response time. In addition, since the ferroelectric liquid crystal display device does not require an active element such as a thin film transistor, production cost is not likely to increase. Furthermore, the ferroelectric liquid crystal display device has an advantage of a large viewing angle, so that it has been promising as a display device with a large capacity, e.g., 2000.times.2000 lines.
There are various problems to be solved for putting the ferroelectric liquid crystal display device into practical use. Among them, it is most important to find a means for realizing high contrast in a simple matrix drive. Regarding this problem, the following methods have been proposed.
(1) A method for using oblique vacuum evaporation
(2) A method for using a high pretilt alignment film
(3) A method for processing using an AC electric field
(4) A method for using a naphthalene compound
(5) A method for using Cl-uniform orientation
(6) A method for using a liquid crystal material having negative dielectric anisotropy
A method (1) is proposed in T. Uemura et al., Proc. SID, 175 (1987). This method requires oblique vacuum evaporation, making it difficult to mass-produce a device and to obtain a device with a large area. Another method (2) is proposed in N. Yamamoto et al., Jpn. J. Appl. Phys., 28, 524 (1989). According to the experiences of the inventors, it is not so easy to obtain uniform orientation in a large area using a high pretilt alignment film. Another method (3) is proposed in Y. Sato et al., Jpn. J. Appl. Phys., 28, L483 (1989) and in H. Rieger et al., Proc. SID, 396 (1991). According to this method, an AC electric field with a low frequency and a high voltage is applied to a conventional ferroelectric liquid crystal cell, whereby a chevron structure in a cell is forced to be changed to a quasi-Bookshelf geometry which is nearly ideal. The method (3) has advantages such as a high contrast, a large memory angle and a bright display; however, there are still problems preventing its practical use, such as increased response time and a change in characteristics with time during driving. Another method (4) is proposed in A. Mochizuki et al., Ferroelectrics., 122, 37 (1991). According to this method, a quasi-Bookshelf geometry which is nearly ideal is obtained using a specific naphthalene compound to realize high contrast. However, naphthalene compounds to be used are limited, so that a great amount of difficulty is assumed for realizing a practical ferroelectric liquid crystal display device with a large screen and a large display capacity, using this method. Another method (5) is proposed in Koden et al., Future Liquid Crystal Display and its Materials: Ferroelectric liquid crystal and antiferroelectric liquid crystal, p. 114 (1992) (under the supervision of A. Fukuda). According to this method, high contrast is obtained employing specific orientation such as Cl-uniform obtained in a parallel rubbing liquid crystal cell using an alignment film with a high pretilt angle. Here, "a parallel rubbing cell" refers to a liquid crystal cell fabricated so that rubbing directions of the upper and lower alignment films are identical. In this method, it is difficult to selectively obtain Cl-uniform orientation in a large area.
Another method (6) is proposed in P. W. H. Surguy et al. , Ferroelectrics, 122, 63 (1991). This method is a promising method for realizing high contrast. A sample of a ferroelectric liquid crystal display device has already been fabricated using this method (P. W. Ross, Proc. SID, 217 (1992)).
Hereinafter, the method (6) will be described in detail.
A conventional ferroelectric liquid crystal material whose dielectric anisotropy is not negative exhibits a .tau.-V characteristic shown in FIG. 1A. More specifically, .tau. (a memory pulse width, or a pulse width required for memory) monotonously decreases with the increase in voltage. In contrast, a ferroelectric liquid crystal material having negative dielectric anisotropy exhibits a .tau.-V (.tau.-V.sub.min) characteristic having a local minimum value V.sub.min shown in FIG. 1B. Surguy et al. have reported a driving method shown in FIG. 2 as a driving method using this characteristic (P. W. H. Surguy et al., Ferroelectrics, 122, 63 (1991)). The principle of this driving method is briefly shown in FIG. 3. According to the principle, when a voltage of .vertline.V.sub.s -V.sub.d .vertline. is applied, memory states are switched in the ferroelectric liquid crystal display device, and when a voltage of .vertline.V.sub.s +V.sub.d .vertline. which is higher than .vertline.V.sub.s -V.sub.d .vertline. and a voltage of .vertline.V.sub.d .vertline. which is lower than .vertline.V.sub.s -V.sub.d .vertline. is applied, the memory states are not switched.
A problem of the method (6) lies in the high driving voltage. According to the report of Ross et al. (P. W. Ross, Proc. SID, 217 (1992)), the driving voltage of a sample of a ferroelectric liquid crystal display device is 55 V. The price of an IC driver for driving the ferroelectric liquid crystal display device goes up with the increase in voltage, so that a high driving voltage necessitates the increase in production cost. In order to fabricate a ferroelectric liquid crystal display device at reasonable price, it is required to drive the display with an inexpensive general-purpose IC driver. That is, it is required that a driving voltage is no more than 40 V. A high driving voltage is due to a high local minimum value V.sub.min in the .tau.-V.sub.min characteristic. Thus, in order to drive a display with a driving voltage of 40 V or less, a ferroelectric liquid crystal display material exhibiting a local minimum value of about 30 V needs to be developed.
According to Surguy et al. (P. W. H. Surguy et al., Ferroelectrics, 122, 63 (1991)), a local minimum value V.sub.min is given by the following equation: ##EQU1## where E.sub.min is a local minimum value of electric field intensity, d is the cell thickness, Ps is spontaneous polarization, .DELTA..epsilon. is dielectric anisotropy, and .theta. is the tilt angle. As is understood from this equation, in order to obtain a lower local minimum value V.sub.min, large negative dielectric anisotropy and small spontaneous polarization are required. On the other hand, the response speed of ferroelectric liquid crystal is proportional to spontaneous polarization, so that it is difficult to obtain quick response when spontaneous polarization is decreased.
In general, the ferroelectric liquid crystal mixture is prepared by adding a chiral compound to an achiral liquid crystal mixture exhibiting a smectic C phase. Therefore, it is required that a chiral compound which can cause high response in a small amount is added to an achiral liquid crystal mixture having low viscosity. A chiral compound generally has high viscosity, so that its added amount is preferably small.
In order to obtain a satisfactory ferroelectric liquid crystal device, the following conventional conditions should also be satisfied.
(1) The ferroelectric liquid crystal material exhibits a chiral smectic C phase in a wide range of temperature around room temperature.
(2) The ferroelectric liquid crystal material exhibits a phase sequence of IAC (Isotropic-Smectic A-Smectic C) or INAC (Isotropic-Nematic-Smectic A-Smectic C) so as to obtain satisfactory orientation properties and bistability.
(3) The helical pitch of the chiral nematic phase and the chiral smectic C phase is sufficiently longer than the cell thickness.
(4) The ferroelectric liquid crystal material has satisfactory chemical stability and optical stability.
If required, the tilt angle, refractive index, specific resistance, and the like can be optimized.